Field of the Invention
The present invention relates to a controller for controlling the spinning of a sample tube used in NMR measurements.
Description of Related Art
In some NMR (nuclear magnetic resonance) measurements, a sample tube holding a sample therein is spun. For example, in solid-state NMR measurements, a sample tube holding a solid sample therein is spun at high speed while tilted at a given angle (magic angle) with respect to the direction of a static magnetic field. Under this condition, a transmit/receive coil surrounding the sample tube generates an RF magnetic field. The resulting NMR signal is detected by the transmit/receive coil.
In the above-described NMR measurements, it is necessary to measure the spinning rate of the sample tube. For example, JP-A-3-229183 discloses an apparatus for measuring the spinning rate of an NMR sample tube. A reflecting plate is mounted on the surface of the sample tube. Light is directed at the reflecting plate. As the sample tube is spun, variations of the level of light reflected from the reflecting plate are detected as a pulse sequence signal. The spinning rate is determined based on the pulse sequence signal. Published Technical Report No. 92-22816 of Japan Institute of Invention and Innovation discloses a method of measuring the spinning rate of a sample tube having a portion reflecting light by illuminating the sample tube with light and detecting variations of the level of reflected light as a pulse sequence signal, the variations being caused concomitantly with spinning of the sample tube.
Japanese Patent No. 2,797,100 discloses a liquid level detector for detecting a liquid level by detecting the difference in static capacitance between air inside a container and a liquid as a digital waveform signal and comparing this digital waveform signal against a reference clock signal.
Where the spinning rate of an NMR sample tube is found based on a pulse sequence signal representing the spinning of the tube, a method as illustrated in FIG. 7 is utilized as an example. A pulse sequence signal 110 shown in FIG. 7 consists of a plurality of pulses arranged in a time-sequenced order (from left to right as viewed in the figure). In relation to this pulse sequence, gate periods Tg of a given duration are established in turn. The number of pulses falling within each gate period Tg is counted. The spinning rate (rotational speed) F is found from the obtained count value Ns. The spinning rate F is given by
                    F        =                  Ns                      M            ·            Tg                                              (        1        )            where M is the number of pulses detected during one revolution of the sample tube.
As shown in FIG. 7, the starting point Ts of one gate period Tg may not be coincident with the timing at which the rising edge 200 of a pulse representing spinning is detected or the ending point Te may not be coincident with the timing at which the falling edge of the pulse is detected because the gate periods are not synchronized with the pulse sequence signal representing spinning. In FIG. 7, the rising edges of the pulses are indicated by upwardly directed arrows, while the falling edges the pulses are indicated by downwardly directed arrows. Since the gate periods Tg and the pulse sequence signal are not synchronized in this way, deviations in time (i.e., errors in time) occur around the beginning and ending of each gate period Tg. An error corresponding to one period, at maximum, may occur. Accordingly, a range in which the true spinning rate F exists is given by Eq. (2). The accuracy at which the spinning rate F is measured is given by Eq. (3).
                              Ns                      M            ·            Tg                          ≤        F        <                              Ns            +            1                                M            ·            Tg                                              (        2        )                                                      1                          M              ·              Tg                                ⁢                                          ⁢          or                ±                  1                      2            ⁢                                                  ⁢                          M              ·              Tg                                                          (        3        )            
The aforementioned errors in time deteriorate the accuracy at which the spinning rate is measured. It is conceivable to increase the duration of each gate period Tg such that the number of detected pulses is increased, for reducing the measurement error. However, if the duration of each gate period Tg is prolonged, the time interval during which the spinning rate is computed is increased and so the time taken until the measurement results are fed back to the spinning control system is lengthened. This deteriorates the responsiveness of the feedback control. Furthermore, local variations in spinning rate cannot be detected. Consequently, feedback control cannot be provided in response to the variations.